mlab_visual.py example issue (issue #1241)

In order to fix the issue in mlab_visual.py:

Add
from tvtk.api import colors

and replace 2 lines
b2 = visual.box(x=4., color=visual.color.red)
b3 = visual.box(x=-4, color=visual.color.red)
by
b2 = visual.box(x=4., color=colors.red)
b3 = visual.box(x=-4, color=colors.red)

Open mlab_visual.py and inspect the color assignments in the example. Add the tvtk colors import and replace the two visual.color.red references with colors.red; done means the example uses the requested TVTK color values.

Written by the indexing model from the issue text.